The present invention relates to filtration modules or elements, and in particular to a new and useful method of producing multi-layer hollow fiber filtration modules. Moreover, the present invention provides a method of producing a variety of multi-layer hollow fiber filtration modules that have performance results that have a predictable performance such that the modules are directly scalable. Specifically, the present invention is directed to an improvement of the invention described in U.S. Pat. No. 5,626,758 hereinafter xe2x80x9cthe Belfort patentxe2x80x9d), incorporated herein by reference in its entirety.
Vortices are well known in the art. Both Taylor and Dean vortices have been harnessed to improve the performance of pressure driven membrane processes. See Winzeler, H. and Belfort, G., xe2x80x9cEnhanced performance for pressure-driven membrane processes: the argument for fluid instabilitiesxe2x80x9d, J. Membrane Sci., 80. 35-47 (1993). Moreover, such vortices have been harnessed with curved tubular membranes. See Srinivasan, S. and Tien, C. xe2x80x9cReverse osmosis in a curved tubular membrane ductxe2x80x9d, Third International Symposium on Fresh Water from the Sea, Vol. 2, pp. 587-600 (1970). The Belfort patent is directed to harnessing Dean vortices in curved hollow fiber membranes to reduce polarization and fouling at the membrane surface, thereby increasing the efficiency of the membrane module.
Dean vortices are formed in curved conduits under laminar flow conditions when the Dean number of the fluid is above a critical Dean number. Vortices exist in turbulent flow, but such vortices are chaotic and excessive pumping energy and/or large diameter tubes are required to generate them. The physical cause behind the formation of the vortices is the centrifugal force exerted on the fluid elements as the liquid flows around the curved surface. This results in the radial motion of the fluid elements in the plane transverse to the principal direction of the flow and the final formation of vortices in that plane. FIG. 1 provides an illustration of Dean vortices in a curved fluid tube, such being known to the prior art.
The Belfort patent is directed to the use of Dean vortices in curved hollow fibers to obtain filter modules with unique performance attributes. The Belfort patent discloses the use of a curved fluid tube constructed to form Dean vortices to improve filtration. The Belfort patent improved upon other prior art modular designs for pressure-driven membrane processes, such as reverse osmosis, and ultrafiltration that were based on maximizing membrane area per unit volume and on the handling convenience of the module.
Indeed, the Belfort patent adds to the many pre-existing methods for reducing concentration polarization (xe2x80x9cCPxe2x80x9d) and fouling, including chemical modification of the membrane surface and physical methods such as scouring. Hydrodynamic methods are also known which rely on eddies during turbulent flow, or induced flow instabilities. Introducing inserts into the flow path can create such induced flow instabilities.
Many of these methods were discussed in the paper by George Belfort entitled, xe2x80x9cFluid mechanics in membrane filtration: recent developmentsxe2x80x9d, J. Membrane Sci., 40, 123-147 (1989). However, these prior art methods, as well as the Belfort patent, fail to provide guidance on producing Dean vortices-producing filtration modules that are directly scalable.
The Belfort patent teaches multi-layered modules. See FIG. 14 of the. Belfort patent. However, it fails to provide any teaching on manufacturing Dean vortices-producing filtration modules that are directly scalable.
In view of the teachings of the Belfort patent, one of ordinary skill in the art would recognize that a pilot or process module, as opposed to a lab scale module, would either require multiple layers of hollow fibers to handle the increased fluid flow, an implausibly long cartridge with fibers wrapped around a single mandrel or a very wide cartridge that has a low packing density with multiple single-layered fiber wrapped mandrels sealed into a tube-sheet. However, as with other prior art uses of flow instabilities, the Belfort patent fails to teach those of skill in the art the ability to directly scale up a lab module to a multi-layered pilot or process (manufacturing) module without requiring significant trial and error. The Belfort patent merely suggests that the invention is amenable to such scaling.
The present invention provides Dean vortices-producing multi-layer filtration modules that are directly scalable. Specifically, the present invention provides methods that use a performance characteristic to develop substantially directly scalable multi-layer filtration modules for filtering solutions that benefit from the de-fouling properties of Dean vortices. More specifically, the present invention includes using a mathematical relationship of Dean number and shear rate to predict the performance of individual layers within a multi-layered module of curved hollow fibers. The present invention allows for performance of each layer within a multi-layered module to be designed to be substantially equivalent (defined below). The present invention describes the means of producing such multi-layered modules.
The present invention is directed to Dean vortices-producing multi-layered filtration modules and methods of designing such modules. Such modules are characterized by each layer having a substantially equivalent performance characteristic. This characteristic allows for the construction of filtration modules that are directly scalable. This is a substantial improvement over the prior art as it allows one to predict the performance of an eleven-layer module, for example, based upon the performance characteristics of a one-layer module.
As provided above, the present invention is also directed to methods of producing the multi-layered filtration modules using a performance characteristic. This performance characteristic is computed via a mathematical function including the Dean number and shear of a liquid. In determining this performance characteristic, the inventors have discovered a means for accurately and precisely estimating the performance of one or more layers of hollow fiber tubing in a filter module designed to use Dean vortices to de-foul the inner surface of the membrane. The inventors can then create subsequent layers having substantially equivalent performance by manipulating one or more of the characteristics of the hollow fiber to be used in such a module. Such hollow fiber characteristics that are contemplated for manipulation include the inner diameter, outer diameter, wall thickness, length, pore size and symmetry of the membrane. Of course, the composition of the hollow fiber can also be adjusted, that is the polymer or polymers used to create the hollow fiber.
For purposes of this invention, the phrase xe2x80x9cde-foulxe2x80x9d or xe2x80x9cde-foulingxe2x80x9d means the removal of species retained on the surface of a hollow fiber including, but not limited to solutes, particulate matter and gels. Inherent with such xe2x80x9cde-foulingxe2x80x9d is that solutions which tend to polarize become less polarized.
While it is within the present invention to produce a multi-layered filtration module with layers of hollow fibers comprised of different materials, this is not preferred. Moreover, it is even less expensive to produce a module of the present invention with fibers of the same material and having the same internal diameter and external diameter. Shortening the fiber length of the hollow fibers used in the subsequent fiber layer of the filter module to produce layers having substantially equivalent performance is the preferred means of producing the filtration modules of the present invention.
The present invention is particularly adapted for use with polarized solutions, preferably polarized aqueous solutions, and therefore includes methods of producing multi-layer filtration modules that use Dean vortices to remove deposits, remove suspended dissolved matter near the membrane and/or direct solutes away from membrane surfaces, the performance of such modules being able to be scaled directly with devices that have different numbers of layers. It is contemplated that the present invention may be used with non-polarized solutions.
With respect to the number of layers, one would preferably make available to customers needing to filter their polarized solvents, it is preferable to produce a three-layer module for pilot scale applications. With respect to process scale applications, eleven-layer modules have been found to be appropriate by the inventors. In addition, it is contemplated that process scale modules will comprise a plurality of multi-layered fiber bundles.
However, the present invention contemplates modules that have four to ten layers or modules with greater than eleven layers. The inventors recognize that with certain applications, it may be desirable to have process-scale modules that are longer than those set forth in the examples below. In such circumstances, fewer layers may be needed.
The multi-layer separation modules of the present invention are deemed to be particularly useful for the different types of filtration that are currently practiced by those of ordinary skill in the art: reverse osmosis, nanofiltration, ultrafiltration, diafiltration, clarification, pre-filtration, microfiltration and the like. The present invention is further contemplated for gas transfer to or from liquids. One use would be the oxygenation of blood. The Dean vortices would enhance transport of the gas to the liquid by reducing the liquid-side mass transfer resistance. The Dean vortices would also enhance any anti-thrombogenetic properties of the membrane surface. The present invention, since it teaches the ability to produce membrane modules that have predictable performance, would have substantial benefits in the oxygenation of blood during heart surgery as the module to module performance would be very consistent and thereby pass the strict FDA requirements for such products. It may also allow for use of less expensive but more porous membrane, such a polyethersulfone, as the Dean vortices can minimize the absorption of blood components to the membrane wall.
The inventors further recognize that the present invention may be useful for heretofore-undiscovered genera of filtration.
With respect to reverse osmosis (RO), it is generally used for separation of salt and low molecular weight organic matter from the liquid carrier. It is contemplated that the present invention will be useful for general RO applications as well as specific applications such as desalination and purification of brackish water and seawater. Another potential use is to desalinate potable water for production of water for injection. The benefit of the present invention for these applications is the minimization of pre-treatment steps, both chemical and physical, prior to RO filtration.
With respect to nanofiltration (NF), it is generally used for fractionation of salt and low molecular weight organic matter. It is contemplated that the present invention will be useful for general NF applications as well as specific applications such as antibiotics processing. This technology, in combination with the appropriate NF hollow fiber, could be used for softening of residential water. Diafiltration would be appropriate for a de-salting or solvent exchange as well with a nanofiltration hollow fiber.
With respect to ultrafiltration (UF), it is generally used for fractionation of medium molecular weight organic matter and transport of salt and low molecular weight organic matter. It is contemplated that the present invention will be useful for general UF applications as well as specific applications such as albumin purification. Another use of ultrafiltration and the present invention is for recovering electrophoretic paint from post-application rinses. Electrophoretic paint is expensive but has substantial performance attributes. Electrophoretic paints can be economically used only if the rinse mixture, created after the painted car is rinsed, is concentrated and the paint removed and reused. This is a highly fouling process and use of a module of the present invention would reduce the amount of fouling and enhance the periodic cleaning of the ultrafiltration module.
With respect to microfiltration (MF), it is generally used for retention of colloids, and other small particles and transport of salt, low molecular weight organic matter and other dissolved organic matter and solutes. It is contemplated that the present invention will be useful for general MF applications as well as specific applications such as cell processing whereby cellular debris is retained but the desired protein is transported through the module. It is further contemplated that the present invention will be useful for vaccine processing, specifically for removal of virus particles and/or cell debris.
In addition to the other MF applications set forth above, it is also contemplated that the present invention will be useful for transgenic milk clarification in which milk colloidal components or micelles are separated from milk proteins and enzymes.
A further MF application contemplated for the present invention is protein precipitate clarification, especially after the protein re-folding processes. Another object of the present invention includes the clarification of a variety of cell broths including lysed cell broths. An object of the present invention would include the clarification of mammalian cell broths such as Chinese hamster ovary (xe2x80x9cCHOxe2x80x9d) cell broths, yeast cell broths and bacterial cell broths such as E. coli. 
Preferably, modules of the present invention will be used for the above MF applications or other applications. With respect to ME applications, it is contemplated that the modules of the present invention would be particularly suited for cellular and colloidal clarification.
With respect to UF applications, it is contemplated that the modules of the present invention may be adapted for high performance tangential flow filtration (xe2x80x9cHPTFFxe2x80x9d) for protein fractionation. HPTFF is described in U.S. Pat. Nos. 5,256,294 and 5,490,937, and the subject matter disclosed therein is incorporated herein by reference in its entirety.
Other separation processes that would benefit from the present invention includes gas separation membrane processes. This would be particularly useful for high flux, highly permeable membranes where the concentration polarization could occur. During gas separation, a concentration gradient builds at the boundary layer. The Dean vortices would prevent the gradient from occurring, thereby increasing flow of gas across the membrane.
With respect to the design of modules, the present invention allows for adjustment of the mandrel length and diameter to suit the desired performance characteristics. Moreover, the winding angle of the hollow fiber may also be adjusted to create or eliminate gaps on the mandrel. Preferably, there are no gaps between the hollow fibers.
Moreover, the present invention provides guidance on selecting the number of fibers one would want to wind, a xe2x80x9cbundlexe2x80x9d, around the mandrel at one time. Furthermore, the present invention even allows the module manufacturer to pre-determine the amount of the mandrel that should be potted, the length of the fibers to be cut for each layer and the number of mandrel units, a mandrel with a specific number of layers and fiber characteristics, that one would want to combine to create a process-scale device.
A further object of the present invention includes the removal of agglomerations and gels from chemical-mechanical planarization (xe2x80x9cCMPxe2x80x9d) slurries used in producing semiconductors.
A further object of the present invention is to provide a compact, relatively short filtration module. This is accomplished, in part, by the fiber bundle of the present invention having a high packing density. A shorter filtration module can be designed more easily by using multiple mandrel units in a single module.
The various features of novelty that characterize the present invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings, descriptive matter and examples in which the preferred embodiments of the invention are illustrated. As it will be realized, the invention is capable of other different embodiments, and its several details are capable of modifications in various, obvious aspects all without departing from the invention. Accordingly, the drawing and the descriptions will be regarded as illustrative in nature and not as restrictive.